MIRROR WITH HEATER

Abstract: A mirror with a heater comprises a mirror base plate, a combined reflecting film and heating resistance film or a reflecting film and a heating resistance film formed on the mirror base plate, and at least a pair of electrodes provided in opposition to each other for electrically energizing the heating resistance film for heating. The combined reflecting film and heating resistance film, the reflecting film and/or the heating resistance film provide a mirror image with a good visibility by heating the surface of the mirror. The electrodes are arranged to heat the entire surface thereof uniformly.

Description: FIELD OF TECHNOLOGY The present invention relates to a mirror with a heater comprising a mirror base plate and heating means. The present invention relates in particular to a mirror with a heater, which has a reflective film-cum-heating resistor film, or a reflection film and a heating resistor film, formed on a mirror base plate and includes at least a pair of electrodes for applying current to the heating resistor film to heat it, and which is suitably used in a bathroom and a vehicle and can prevent its surface from being clouded with moisture, rain droplets, dew or ice. BACKGROUND TECHNOLOGY When a vehicle are traveling in rainy or snowy weather, the outside mirrors are clouded with water droplets or ice, degrading the rearward view and therefore lowering the safety of driving. To prevent this, various types of mirrors have been proposed, which can be heated to remove water droplets and ice adhering to the mirror surface. For example, Japanese Utility Model Publication No. 58-28937/1983 discloses a mirror for a vehicle, in which a heat distribution plate with high heat conductivity is attached to the back of a mirror base plate and has a heating body bonded to the back of the heat. Further, Japan Utility Model Publication No. 62-33648/1987 discloses a mirror with heater, in which a flat heater is fixed to the back of a mirror body and the pattern of the heater is made more dense in the peripheral portion of the mirror than in the center. Further, Japanese Utility Model Publication No. 102599/1992 discloses a flat heating body for a mirror, in which a heating region is divided into sections by electrodes. The above-mentioned mirror and flat heating body for a mirror adopts a structure in which an electric heating plate which has a complex heating resistor pattern or a complex electrode pattern is fixed to the back of the mirror base plate in order to heat the entire mirror surface evenly to provide a good view. By the method using the electric heating plate, which is provided separately from the mirror base plate, it is necessary to design and manufacture a complex heating resistor pattern and electrode pattern, which increases the cost. Another drawback of this method is that because the mirror base plate is heated through the conduction of heat from the separate electric heating plate. the heat efficiency is low and it takes a long time to remove water droplets. To solve the above problems, Japanese Utility Model Laid-Open No. 5-13872/1993 proposes a mirror with a heater. in which chromium or nichrome is deposited on the surface of the mirror base plate by vacuum vapor deposition or sputtering to form a reflective heating resistor film, whose surface is coated with an insulating overcoat layer. JP-U-61 192963 discloses a sheet-like heating element 1 on a back side of a glass 2. JP-B-63 66034 discloses a coloured mirror having a metal heating film and a coloured thin film of a metal oxide on a glass substrate. Combination of the metal (titanium) oxide layer and the titanium layer provides a reflective function and the titanium layer provides a heating function. A coloured film 2 is disposed between the metal heater film 3 and the glass base plate. JP-B-1 24095 discloses a coloured mirror having a glass plate 1, opaque layer 2 on one side of the glass plate 1, and multi-layer coating portion 10 having at least one electrically conductive layer of different refractive index. A reflecting function is produced by a plurality of layers and a heating function is produced by a single layer. JP-U-60 195258 discloses a window glass for a vehicle having a transparent conductive film for presenting a heating function. JP-U-62 112632 shows a mirror having a temperature indicator on a surface thereof. These documents represent the closest prior art. Ordinary mirror reflection films are made of such materials as aluminium and chromium, deposited by vacuum vapor deposition and sputtering. It is, however, difficult to use an aluminium or chromium film as the reflective film-cum-heating resistor (reflective heating resistor film) of the mirror with a heater. The reason for this is that the electrical resistivity of aluminium and chromium is low. That is, a film made of aluminium or chromium has a low resistance, which allows a large current to flow, increasing the power consumption and making the temperature control difficult. One possible method of solving this problem is to raise the resistance of the film made of aluminium or chromium, that is, to reduce the thickness of the aluminium or chromium film formed as the reflective heating resistor film as much as possible. When a mirror with a heater is used for a vehicle, the current applied to the mirror is preferably in a range of 1 to 5A. If the current is under this range, the mirror may lack the ability to melt ice in the cold season, especially when exposed to wind; and if the applied current is over this range, the current application by temperature control function may result in overheat due to overshoot, burning of peripheral components and even a human. Considering the fact that in the case of vehicles a voltage of DC 12 V is applied to a mirror with a heater, the sheet resistance of the reflective heating resistor film of the mirror is preferably in the range of 4-20 Ω /square to enable uniform heating of the mirror irrespective of its shape. Considering the above, it is therefore possible to use aluminum or chromium for the heating resistor of the mirror with a heater for vehicles if the film thickness is set below 0.01 µm when aluminum is used for the reflective heating resistor film and if the film thickness is set below 0.03 µm when chromium is used. Such a thin film, even though the film is made of metal, transmission of light through the film cannot be ignored and the mirror works as a half-mirror rather than as a reflective mirror, raising a problem that depending on how light falls on to the mirror, the back side may be seen through the film, degrading the view of vision of the mirror. Further, though electrodes for applying current and heating the reflective heating resistor film are attached to the film, but the adhesion of the chromium film to the electrodes is poor. Another method of solving the above problem may be to use a material for the film which has a higher electrical resistivity than aluminum and chromium. Materials with high electrical resistivity include silicides such as nichrome, chrome silicide and titanium silicide. Nichrome, however, has a poor adhesion to electrode materials and consequently it is hard to achieve a stable performance. The chromium silicide film needs to be at least about 1 µm thick to conduct a desired heating current but the film itself easily cracks due to stresses and the mirror base plate such as of glass may break during heating. This phenomenon is particularly noticeable in a concave mirror in which residual bending stress remains in the glass plate. Moreover, silicides generally have a low reflectivity (reflection factor) of around 30%, and at such a low level of reflectivity the function as a reflection film of the mirror cannot be fulfilled. Further, the heating resistor is restricted by its temperature coefficient of resistance. When the temperature coefficient of resistance is too large, the heater resistance increases with an increasing temperature and reduces the current, it takes a long time for the mirror to be heated to a desired temperature, making it impossible to completely remove water droplets and ice. When on the contrary the temperature coefficient of resistance is too small, the current application by temperature control function may result in overheat due to current overshoot, burning peripheral components and even humans. When a reflective heating resistor film is formed on the surface of the mirror base plate, only the central part of the mirror is easy to heat. For uniform heating of the entire mirror surface, conventionally the electrodes are provided near the peripheral portion of the mirror base plate. This method is often not effective. Mirrors for cars generally have a mirror base plate of a figure, not a circle nor rectangle, but generally parallelogram, trapezoid, oval and diamond having a narrow angle portion whose interior angle defined by the edges of the mirror base plate is small and a wide angle portion whose interior angle is large. When such a mirror base plate is used, the wide angle portion is more likely to be heated. To quickly remove water droplets in the narrow angle portion that is difficult to heat, a large amount of electricity is required. Not only is this inefficient but it may also overheat the wide angle portion, burning and deforming peripheral components such as resin holders and even burning a human when he or she touch the mirror. As described above, the mirror with a heater disclosed in Japanese Utility Model Laid-Open No. 13872/1993 does not meet the expectations in quality. DISCLOSURE OF INVENTION An object of this invention is to provide a mirror with a heater which has an appropriate reflectivity and can form a clearly recognizable mirror image and whose surface temperature can be controlled and raised to quickly remove water droplets or ice adhering thereto. Another object of this invention is to provide a mirror with a heater whose the entire surface of the mirror base plate can be heated uniformly, making it possible to control the temperature, and quickly removing water droplets or ice adhering thereto. According to the present invention a mirror with a heater comprises a mirror base plate (1) and heating means, characterised in that : said heating means has a reflecting heating resistor film (2) disposed directly on the mirror base plate (1) for providing a reflective surface on the mirror base plate (1) and uniformly heating the reflective surface, the reflective heating resistor film being made of titanium; and that said mirror comprises at least one pair (3a, 3b) of opposing electrodes disposed on the reflective hating resistor film (2) for applying electricity to the reflective surface, wherein a maximum voltage drop in the electrodes (3a, 3b) with respect to a current feeding point on the electrodes is between 0.5% and 20% of a supply voltage. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic perspective view showing the back of a first embodiment; Figure 2 is a schematic vertical cross section of the mirror shown in Figures 1, 3 and 5; Figures 3 to 5 are schematic perspective views showing examples of the back of a mirror according to the present invention; Figure 6 is a schematic vertical cross section of an example of a mirror according to the present invention; Figures 7 to 10 are schematic perspective views showing examples of the back of a mirror according to the present invention; Figure 11 is a sheet resistance distribution diagram of a mirror according to the present invention; Figures 12 to 14 are schematic perspective views showing examples of the back of a mirror according to the present invention. Figure 1 is a schematic perspective view showing the back of a mirror with a heater, used as a vehicle door mirror, and Figure 2 is a schematic vertical cross section of Figure 1 . Reference numeral 1 represents a mirror base plate made of such a transparent material as glass. On the back of this mirror base plate 1 is formed a reflective heating resistor film 2, which is a titanium film deposited by sputtering or vacuum vapor deposition. The titanium film referred to here is formed by sputtering or vacuum vapor deposition and therefore includes titanium films containing a trace amount of impurity depending on the condition and equipment employed in the manufacturing process. The impurity may include oxygen, nitrogen and carbon. and their contents are up to 10 atomic percent for oxygen, up to 1 atomic percent for nitrogen and up to 5 atomic percent for carbon. The titanium film preferably has a thickness in a range of 0.05-0.15 µm depending on the shape of the mirror. Further, provided on the back of the reflective heating resistor film 2 are a pair of opposing electrodes 3a, 3b for applying electricity to the reflective heating resistor film 2. To uniformly heat the entire surface of the mirror, the interval of the electrodes 3a, 3b near the corners of the mirror base plate 1 is narrower than that at the central part. These electrodes 3a, 3b can be formed by a variety of methods. For example, a copper paste or silver paste may be used to form a thin layer of copper or silver, and solder is applied to the layer. Alternatively, a thin film of nickel or gold is formed by nickel or gold plating and the plating layer is used as electrodes. For electric insulation, the back of the mirror is coated with an insulating material, such as resin or rubber, which has such a low Young's modulus that the coating does not crack when subjected to temperature change. Reference numeral 5 represents lead wires connecting the electrodes 3 and a power supply circuit (not shown). Reference numeral 6 denotes a temperature control element for control the heating. The mirror with a heater was fabricated as follows. On the mirror base plate 1 of glass a titanium film is deposited to a thickness of 0.1 µm by sputtering to form a reflective heating resistor film 2. When a DC voltage of 12 V was applied across the mirror, a current of 4 A flowed. When the heating of the mirror was controlled by a temperature control circuit having a thermistor as a temperature detector or by a thermostat, the temperature of the mirror surface was able to be controlled in a range of 50-60°C as set beforehand. The mirror had a reflection factor of 45-50%, which was slightly lower than that of a conventional chromium reflection film, but it can be, used as a mirror without raising any problem. Also, it did not cause a problem that the back of the mirror was seen irrespective of the way the light struck the mirror. Further, other problems that the film cracked due to stress and that the glass plate forming the mirror base plate was broken during heating, did not occur. Embodiment 1 Figure 3 is a schematic perspective view showing the back of a mirror with a heater mounted on a vehicle door. Figure 2 is a schematic vertical cross section of the mirror. Reference numeral 1 is a mirror base plate made of a transparent material such as glass. The back of the mirror base plate 1 is formed with a reflective heating resistor film 2. The back of the reflective heating resistor film 2 is provided with a pair of opposing electrodes 3a, 3b to apply electricity to the film. To heat the left and right side portions of the mirror (in Figure 3 ), the opposing electrodes 3a, 3b are so arranged that the interval between the electrodes 3a, 3b along the left and right sides of the mirror base plate 1 is narrower than the electrode interval at the central portion. These electrodes 3a, 3b can be formed in a variety of ways, as mentioned earlier. Though the electrodes are normally formed to a uniform thickness and to a uniform width, it is possible to make the thickness and width of the electrodes uneven to change the resistance of the electrodes depending on the locations or to connect electrodes of two or more different materials to change the rate of voltage drop in the electrodes. Further, the number of electrodes is not limited to two and, for example, another electrode may be added intermediate between the electrodes 3a, 3b in Figure 3 , using the electrodes 3a, 3b as anodes and using the added electrode as a cathode. Further in Figure 3 another pair of electrodes may be added along the left and right sides of the base plate. Furthermore, for electric insulation and corrosion resistance, the back of the reflective heating resistor film 2 and the back of the electrodes 3a, 3b are coated with an insulating material 7, such as resin and rubber, which has such a low Young's modulus that the coating does not crack when subjected to temperature change. Reference numeral 5 represents lead wires to connect the electrodes 3a, 3b and a power supply circuit (not shown). The lead wires 5 are connected by, say, soldering to the electrodes 3a, 3b. A connection point A 1 of the lead wire 5 and the electrode 3a represents a power feeding point for the electrode 3a; and a connection point A2 of the lead wire 5 and the electrode 3b represents a power feeding point for the electrode 3b. The voltage between the electrodes 3a, 3b drops more away from the feeding points A 1 , A 2 . Hence, end portions E 1 , E 2 of the electrode 3a represent maximum voltage drop points in the electrode, and similarly end portions E 3 , E 4 of the electrode 3b represent maximum voltage drop points in the electrode. In the maximum voltage drop points in the electrode, the maximum voltage drops need to be in a range of 0.5-20% of the supply voltage. When the maximum voltage drops are less than 0.5% of the supply voltage, the amount of heat produced by the electrodes is too small to evenly heat the entire surface of the mirror base plate including the electrodes. Contrarily, when the amount of maximum voltage drop exceeds 20% of the supply voltage, heating the entire mirror requires applying a large amount of electricity, resulting in a low efficiency, loss of electrodes, or cracks in glass. Two or more power feeding points may be provided in each electrode. In Embodiment 1 , the reflective heating resistor film 2 is a titanium film 0.05 µm thick, on which electrodes 3 of copper thin film are deposited by screen printing. When a DC voltage of 12 V was applied between the feeding points A 1 and A 2 of the mirror, a current of 2.0 A flowed. In the mirror of this embodiment, although the temperature was slightly higher at the current feeding points than other portions, the temperature of the mirror surface including portions corresponding to the electrodes was able to be controlled in a range of 45-65°C as set beforehand. Embodiment 2 A mirror with a heater of this embodiment was fabricated in a similar way to that of Embodiment 1 , except that the thickness of the electrodes was made larger. The current between the electrodes was 2.1 A. In the mirror of this embodiment, the temperature of the mirror surface including those portions corresponding to the electrodes was able to be controlled in a range of 50-60°C as set beforehand. Embodiment 3 A mirror with a heater of this embodiment was fabricated in a similar way to that of Embodiment 2 , except that a reflective heating resistor film 2 was formed of titanium and had a thickness of 0.1 µm, and the electrodes 3 are made of silver. The current between the electrodes was 4.1 A. In the mirror of this embodiment, the temperature of the mirror surface including those portions corresponding to the electrodes was able to be controlled in a range of 50-60°C as set beforehand. Embodiment 4 A mirror with a heater of this embodiment was fabricated in a way similar to that of Embodiment 3 , except that a reflective heating resistor film 2 was formed of nichrome and had a thickness of 0.2 µm thick. The current between the electrodes was 3.7 A. In the mirror of this embodiment, the temperature of the mirror surface including those portions corresponding to the electrodes was able to be controlled in a range of 50-60°C according to the setting. Embodiment 5 The mirror of this embodiment was made in a way similar to that of Embodiment 1, except that a titanium film was deposited on the 0.05-µm-thick nichrome film to a thickness of 0.05 µm to form a reflective heating resistor film 2, a thin copper film was formed on the thin silver layer to form electrodes 3, and that a thick solder film was formed on the electrodes. The current between the electrodes was 2.9 A. In this mirror of this embodiment, although the temperature rise was slightly large particularly at around E 1 , E 4 , the temperature of the mirror surface including portions corresponding to the electrodes was able to be controlled in a range of 50-65°C as set beforehand. The voltage drops between A 1 -E 2 and A 2 -E 3 were less than 0.5% of the supply voltage, but because the distances of A 1 -E 2 and A 2 -E 3 were short, these portions were also heated evenly. Embodiment 6 Figure 4 is a schematic perspective view showing the back of Embodiment 6 This embodiment is similar to Embodiment 1, except that each electrode has two feeding points. In Embodiment 6, the feeding points for the electrode 3a are points A 1 and A 3 , and the maximum voltage drop points in the electrode 3a are points E 1 and E 2 , which are the ends of the electrode 3a, and a point E 5 which is a potentially intermediate between the feeding points A 1 and A 3 . The feeding points for the electrodes 3b are points A 2 and A 4 , and the maximum voltage drop points in the electrode 3b are points E 3 and E 4 which are the ends of the electrode 3b, and a point Eg which is a potentially intermediate between the feeding points A 2 and A 4 . In Embodiment 6, on the mirror base plate of glass was formed a chromium layer 0.02 µm thick by sputtering. On this chromium layer, a titanium layer is formed by sputtering to a thickness of 0.03 µm to use as a reflective heating resistor film, on which a silver thin film was formed by screen printing using silver paste. On this thin silver layer a copper thin film was deposited to form electrodes. A DC voltage of 12 V was applied between the feeding points A 1 , A 3 and A 2 , A 4 . The current between the electrodes was 4.5 A. In the mirror of this invention, although the temperature rise was slightly large at the electrode end portions, particularly near points E 1 , E 4 , the temperature of the mirror surface including those portions corresponding to the electrodes was able to be controlled in a range of 50-65°C as set beforehand. The voltage drops between A 1 and E 2 and between A 2 and E 3 were less than 0.5% of the supply voltage but because the distances between A 1 and E 2 and between A 2 and E 3 were short, these portions were also evenly heated. Measurements were made of voltage drops between the maximum voltage drop points in the mirror of Embodiments 1 to 6. The results of the measurement are shown in the table below. Voltage drop (V) (lower row: % of supply voltage) A 1 - E 1 A 1 - E 2 A 2 - E 3 A 2 - E 4 A 1 , A 3 - E 5 A 2 , A 4 -E 6 Embodiment 1 2.0 1.6 1.4 2.2 - - 16.7 13.3 11.7 18.3 - - Embodiment 2 0.9 0.5 0.6 0.8 - - 7.5 4.2 5.0 6.7 - - Embodiment 3 0.6 0.2 0.3 0.7 - - 5.0 1.7 2.5 5.8 - - Embodiment 4 0.3 0.2 0.1 0.3 - - 2.5 1.7 0.8 2.5 - - Embodiment 5 0.2 <0.05 <0.05 0.2 - - 1.7 <0.4 <0.4 1.7 - - Voltage drop (V) (lower row: % of supply voltage) A 1 - E 2 A 3 - E 1 A 2 - E 3 A 4 - E 4 A 1 , A 3 -E 5 A 2 , A 4 - E 6 Embodiment 6 <0.05 0.1 <0.05 0.09 0.08 0.07 <0.4 0.8 <0.4 0.8 0.7 0.6 A number of examples will now be described relating to possible alternative electrode configurations and mirror shapes suitable for use in the present invention. Figure 5 is a schematic perspective view showing the back of a mirror with a heater used as a vehicle door mirror. Again, Figure 2 is a schematical vertical cross section of the mirror. Reference numeral 1 is a mirror base plate made of a transparent material such as glass. On the back of the mirror base plate 1. a reflective heating resistor film 2 of titanium, chromium or nichrome was formed by sputtering or vacuum vapor deposition. The reflective heating resistor film 2 may have a different in structure from that of this embodiment in which the film formed on the back of the mirror base plate 1 serves both as the reflection film and the heating resistor film. For example, a multilayer film may be formed, each of the layers having two functions of a reflection film and a heating resistor film. It is also possible to form an insulating layer between the reflection film and the heating resistor film to electrically isolate them from each other. When a multilayer film is formed, the first layer may be made of aluminum, chromium. nickel, nichrome alloy, or nickel-phosphorus by sputtering, vacuum vapor deposition and plating. The second layer may be formed of titanium, titanium silicide, chromium silicide, tantalum nitride, titanium carbide, tungsten carbide, niobium boride, or iron-chromium-aluminum alloy by sputtering, vacuum vapor deposition or plating. When a reflection film and a heating resistor film are formed separately, the material of the reflective film is aluminum, chromium, nickel, nichrome alloy, or nickel-phosphorous, and the film is formed by sputtering, vacuum vapor deposition or plating; the material of the insulating layer is silica; and the material of the heating resistor film is titanium, titanium silicide, chromium silicide, tantalum nitride, titanium carbide, tungsten carbide, niobium boride. or iron-chromium-aluminum alloy, and the film is formed by sputtering, vacuum vapor deposition or plating. Further, the back of the reflective heating resistor film 2 was provided with a pair of opposing electrodes 3a, 3b to apply electricity to the film. The opposing electrodes 3a, 3b were arranged in such a way that the electrode intervals d 1 , d 2 near the corners of the mirror base plate 1 were narrower than the electrode interval D 1 at the central part. These electrodes 3a, 3b can be formed by a variety of methods, as mentioned earlier. The back of the mirror was coated with an insulating material 7 such as resin for electric insulation. Reference numeral 5 represents lead wires to connect the electrodes 3 and the power supply circuit (not shown). Reference numeral 6 designates a temperature control element for the control of heating. In such a heating resistor film described above, the resistance of the central part of the mirror generally tends to be smaller than those of the corner parts and thus the central part is easily heated. By forming the heating resistor film in such a way that the electrode intervals d 1 , d 2 near the corners of,the mirror are narrower than the electrode interval D 1 at the central part, as in this embodiment, it is possible to heat the corner portions and the central portion equally. Hence, water droplets can be removed evenly from the entire mirror surface without having to apply an excessive power. In the mirror of this invention, the corner portion of the mirror on the side connected to the lead wires 5 is difficult to heat because a greater amount of heat is conducted to the lead wires 5 from this side than from the opposite side. Hence, by setting the electrode interval at the corner portion of the mirror on the lead wire connection side narrower than the electrode interval at the opposite side, it is possible to achieve uniform heating of the mirror. Figure 6 shows an example of a mirror which is similar to the example shown in Figure 5 except that the electrode intervals d 1 , d 2 , narrower than the electrode interval D 1 at the center of the mirror, represent the distances between the opposing, mirror, represent the distances between the opposing. inwardly projecting portions of the electrodes located near the corners of the mirror base plate 1. The effects of this arrangement is similar to that of the embodiment 3. Figure 7 shows an example in which opposing two pairs of electrode portions of which the intervals C 1 , C 2 are smaller than the electrode interval D 1 at the central part of the mirror base plate 1, are provided other than the corner portions of the mirror base plate 1 of Embodiment 10. The advantage of this example is similar to that of the example shown in Figure 6 and is particularly remarkable when the mirror shape is close to a rectangle or parallelogram along long sides of which the electrodes are formed. Figure 8 shows an example in which electrodes 3a, 3b are provided along the opposing long sides of the mirror base plate 1, and another electrode 3c is provided between these electrodes 3a, 3b, the electrodes 3a, 3b being positive and the electrode 3c negative. In the relation between the electrode 3a and the electrode 3c, the electrode intervals d 1 , d 2 along the short sides are narrower than the electrode interval D 1 at the central portion. In the relation between the electrode 3b and the electrode 3c, the electrode intervals d 2 , d 4 , along the short sides are narrower than the electrode interval D 2 at the central portion. The advantage of this example is similar to that shown in Figure 5 and is particularly great when the mirror shape is close to a square or diamond. Figure 9 shows an example in which the mirror is shaped in a circle or an oval and in which two pairs of opposing electrodes 3a, 3b, 3c and 3d are so arranged that the electrode intervals d 1 , d 2 , d 3 , d 4 between the adjacent ends of the electrodes 3a to 3d are narrower than the electrode intervals D 1 , D 2 , D 3 , D4 along two diameters or the major or minor axes. The advantage of this example is similar to that shown in Figure 5. The following example is an example where the sheet resistivity of the heating resistor film is distributed in such a way that the portions of the heating resistor film which have been difficult to heat in the conventional mirror have small sheet resistivities, thereby passing a greater amount of heating current through these portions, enhancing the amount of heat generated and realizing an efficient heating of the entire surface of the mirror base plate. Figure 10 is a schematic perspective view showing the back of a mirror with a heater used as a vehicle door mirror. Reference numeral 1 represents a mirror base plate made of a transparent material such as glass. On the back of the mirror base plate 1 is formed a reflective heating resistor film 2 having an uneven sheet resistivity distribution in the surface. The ununiform distribution of sheet resistivity in the heating resistor film may be such that the sheet resistivity is maximum at the central part of the mirror base plate and minimum near the short sides, or conversely it is minimum at the central part and minimum near the sides. It should be noted that the positions where the sheet resistivity becomes maximum or minimum are not limited to the central part or side parts of the mirror base plate but may be other positions within the mirror base plate. That is, the areas in the mirror base plate where the sheet resistivity is maximum or minimum are set so that portions of the mirror base plate whose temperature, in an even sheet resistivity distribution, would easily rise have large resistances and that portions of the mirror base plate whose temperature, in an even sheet resistivity distribution, would hardly rise have small resistances, thereby permitting quick and uniform heating of the entire surface of the mirror base plate. A variety of methods can be employed to give the heating resistor film an uneven sheet resistivity distribution. For example, the thickness of the heating resistor film may be changed or a plurality of materials with different resistances may be used to form a mosaic-like heating resistor film. In this example titanium is deposited on a generally rectangular mirror base plate 1 of glass by magnetron sputtering to form a reflective heating resistor film 2 with a sheet resistivity distribution such that the sheet resistivity is smaller at peripheral portions of the mirror base plate 1 than at the central portion. The reflective heating resistor film 2 of titanium is formed by a magnetron sputtering technique in which a target (cathode) and a mirror base plate 1 are arranged so that an erosion area where the film is formed at a maximum speed corresponds to the peripheral portion of the mirror base plate 1, and the distance between the mirror base plate 1 and the cathode is small. The thickness of the central portion of the mirror base plate 1 is therefore smaller than that of the peripheral portion. The distribution of the sheet resistivity in the reflective heating resistor film 2 of titanium is shown in Figure 11. The sheet resistivity of the central part was about 1.7 times higher than that of the peripheral part. The sheet resistivity is measured by a four-probe method and the values are converted into relative values to draw the curve. Copper thin layers were formed along the long sides of the mirror base plate 1. thus providing a pair of opposing electrodes 3. Lead wires 5 are connected to the current feeding points A 1 , A 2 on the electrode wires 3a, 3b of the electrodes 3. In this way a mirror with a heater was fabricated. The heating of this mirror was controlled by a temperature control element (thermostat) 6. The surface temperature of the mirror base plate 1 including the peripheral portions was able to be controlled in a 50-65°C range as set beforehand. Figure 12 is a schematic perspective view showing a further example of the back of a mirror with a heater used as a vehicle door mirror. Reference numeral 1 is a generally parallelogram-shaped mirror base plate made of a transparent material such as glass. Out of the four rounded corners of the mirror base plate 1. two corners are narrow angle portions 1b, 1c whose interior angles defined by the sides of the mirror base plate 1 are small, and the other two are wide angle portions 1a, 1d with large interior angles. On the back of the mirror base plate 1 is formed a reflective heating resistor film 2. The back of the reflective heating resistor film 2 is provided with a pair of opposing electrodes 3a. 3b that extends in two directions to the narrow and wide angle portions of the mirror base plate 1 to supply electricity to the reflective heating resistor film 2. These opposing electrodes 3a, 3b are so arranged that the distance between them is narrower near the ends than at the central portion in order to heat the side portions of the mirror. In the electrode 3a, Eb designates an electrode end on the narrow angle portion 1b side of the mirror base plate 1; and Ea designates an electrode end on the wide angle portion 1a side of the mirror base plate 1. In the electrode 3b, Ec designates an electrode end on the narrow angle portion 1c side of the mirror base plate 1; and Ed designates an electrode end on the wide angle portion 1d side of the mirror. In the electrodes 3a, 3b, to make the voltage drop at the electrode ends Eb, Ec on the side of the narrow angle portions 1b, 1c of the mirror base plate 1 with respect to the feeding points A1, A2 lower than the voltage drop at the electrode ends Ea, Ed on the side of the wide angle portions 1a, 1d, the feeding points A1, A2 may, for example, be located on the narrow angle side with respect to the center of the electrodes 3a, 3b, or the electrodes on the narrow angle portions 1b, 1c sides with respect to the feeding points A1, A2 may be made wider or thicker than the electrodes on the wide angle portions 1a, 1d sides or may be formed of materials with lower resistivity than the electrodes on the side of the wide angle portions 1a, 1d. In this example titanium was deposited by sputtering on the glass mirror base plate 1 to a thickness of 0.05 µm to form a reflective heating resistor film 2, on which a copper paste was applied by screen-printing to form electrodes 3 of a thin copper layer with an even resistance distribution. Lead wires 5 were connected to the feeding points A1, A2. which were located on the narrow angle portions 1b, 1c sides with respect to the center of the electrodes 3a, 3b. In this way, the mirror was fabricated. When a DC voltage of 12 V was applied between the feeding points A1 and A2, a current of 2.3 A flowed. At this time, the voltage drops between the feeding point A1 and the narrow angle portion side electrode end Eb, between the feeding point A1 and the wide angle portion side electrode end Ea, between the feeding point A2 and the narrow angle portion side electrode end Ec, and between the feeding point A2 and the wide angle portion- side electrode end Ed were 0.35 V, 0.72 V, 0.34 V, and 0.75 V, respectively. The voltage drop at the narrow angle portion side electrode end of the mirror base plate with respect to the feeding point was smaller than the voltage drop at the wide angle portion side electrode end, and less than 50%. The heating of the heater-incorporated mirror was controlled by a thermostat. Although the temperature near the narrow angle portion of the mirror base plate was slightly low, the mirror surface temperature was able to be controlled in a range of 45-65°C according to the setting. Figure 13 shows a further example of the back of a mirror with a heater used as a vehicle door mirror. Reference numeral 1 represents a generally parallelogram-shaped mirror base plate made of a transparent material such as glass. Out of the four rounded corners of the mirror base plate 1, two corners are wide angle portions 1a, 1d whose interior angles defined by edges of the mirror base plate 1 are large, and the other two corners are narrow angle portions 1b, 1c with smaller interior angles. On the back of the mirror base plate 1 is formed a reflective heating resistor film 2. The back of the reflective heating resistor film 2 is also provided with electrodes 3a, 3b to apply electricity to the film 2. These electrodes 3a, 3b extend in two directions toward the narrow and wide angle portions of the mirror base plate 1. The opposing electrode 3a, 3b are provided with projections at the corners, namely, wide and narrow angle portions to narrow the intervals between the electrodes at and near the short side portions than that at the central portion so as to improve the heating of the short side portions of the mirror base plate 1. A projection ea on the wide angle portion side and a projection eb on the narrow angle portion side of the electrode 3a face a projection ec on the narrow angle portion side and projection ed on the wide angle portion side, respectively. The projections ea, ed on the wide angle portion sides are so shaped as to limit the current concentration at the wide angle portions. A variety of ways are usable for limiting current concentrations on the wide angle portion side projections. Some examples will be described below. (1) Projections are formed on the wide angle portion sides, and not on the opposing narrow angle portion sides. (2) When projections are formed on the wide angle portion sides and the narrow angle portion sides and when the ends of the projections are linear, the current is more likely to concentrate on the ends as the widths of the ends become narrow. Hence, by making the widths of the projections formed at the electrode were ends on the wide angle portion sides wider than the widths of the projections formed at the opposing electrode ends on the narrow angle portion sides, it is possible to limit the current concentration on the wide angle portions. (3) When projections are formed on the wide angle portion sides and the narrow angle portion sides opposing the wide angle portion sides and when the ends of the projections are curved, the current concentration becomes intense as the radius of the arc of the curve becomes small. Hence, by making the radii of the projections formed at the electrode ends on the wide angle portion sides larger than the radii of the projections formed at the electrode ends on the narrow angle portion sides, it is possible to suppress the current concentration on the wide angle portions. (4) When the radii of the projections on the wide angle portion sides and the opposing narrow angle portions sides are equal, the current concentration becomes small as the distance from the end surface of the mirror base plate to the inflection point (vertex) increases. Therefore, by making the lengths from the end surface to the vert of the projection formed at the electrode end on the wide angle portion side larger than the length of the projection formed at the opposing electrode end on the narrow angle portion side, it is possible to suppress the current concentration on the wide angle portion side. With the electrode ends shaped as described above, the concentration of currents flowing into the wide angle portions, which are easily heated, can be reduced, permitting uniform heating of the entire surface of the mirror. In this example, a titanium film was formed by sputtering on a generally parallelogram-shaped curved-surface glass mirror base plate 1 (R=1,400 mm) to a thickness of 0.1 µm to form a reflective heating resistor film 2. Further, the back of the reflective heating resistor film 2 was provided with electrodes 3a, 3b to apply electricity to the film 2. The electrodes 3a, 3b extended in both directions to the narrow angle portions and the wide angle portions of the mirror base plate 1. Current feeding points A1, A2 on the electrodes 3a, 3b were connected with lead wires 5, thus fabricating the heater-incorporated mirror. The projections of these electrode ends are formed linear at their tips, and the widths of the projections ea, ed on the wide angle portion side need to be larger than the widths of the opposing projections ec, eb on the narrow angle portion side. This is to allow uniform heating of the entire mirror surface by reducing the densities of currents flowing into the wide angle portions, which are easily heated. The ratios of the widths of the projections at the wide angle portions to the widths of the projections at the narrow angle portions vary depending on the size of the mirror, and on the material and size of the electrodes. It is preferable that the ratios are increased as the angles of the wide angle portions increase. The heating of the mirror was controlled by a temperature control element (thermostat) 6. The surface temperature of the mirror base plate was able to be controlled in a range of 50-65°C as set beforehand. Figure 14 shows an example in which a temperature detection element is provided near an electrode end on the wide angle portion side of opposing electrode to realize easy temperature control of a portion that is easily overheated in the conventional mirrors, and in which the easily overheated portion is given an increased heat capacity to substantially suppress the temperature rise speed of the portion so that appropriate heating of the narrow angle portion, which has been difficult to heat, will not result in an excessive temperature rise of the wide angle portion, thereby ensuring efficient uniform heating of the entire surface of the mirror base plate. In the vehicle door mirror shown in Figure 14 , titanium was deposited by sputtering over a glass mirror base plate 1 to a thickness of 0.08 µm to form a reflective heating resistor film 2, on which a copper paste was applied by screen-printing to form a thin copper layer as electrodes 3a. 3b with an even resistance distribution. A temperature detection element 6 of thermostat was installed near an electrode end Ea on the wide angle portion side of the opposing electrodes. Current feeding points A1, A2 located on the narrow angle portions 1b, 1c sides from the centers of the electrodes 3a, 3b were connected with lead wires 5. In this way, a mirror with a heater was fabricated. When a DC voltage of 12 V was applied between the feeding points A1 and A2, a current of 3.6 A flowed. The heating of the mirror was controlled by the temperature detection element 6 of thermostat. The temperature of the mirror surface was able to be controlled in a range of 50-65°C as set in advance. In this embodiment. the electrode ends on the wide angle portion sides of the opposing electrodes are denoted by Ea and Ed. Although the temperature detection element 6 may be installed near either electrode end Ea or Ed on the wide angle portion sides, it is preferably placed on the wider angle portion side. It is also possible to use a temperature detection element 6 out of contact with the mirror surface. For example, a temperature detection element 6 comprising an infrared light receiving element may be attached to the mirror holder and the surface of the heating resistor film 2 close to the electrode end Ea or Ed on the wide angle portion side of the base plate with respect to the current feeding point A1 or A2 may be made an infrared ray monitoring portion. In this way the temperature control may be performed.

Claims: A mirror with a heater comprising a mirror base plate (1) and heating means, characterised in that : said heating means has a reflecting heating resistor film (2) disposed directly on the mirror base plate (1) for providing a reflective surface on the mirror base plate (1) and uniformly heating the reflective surface, the reflective heating resistor film being made of titanium; and that said mirror comprises at least one pair (3a, 3b) of opposing electrodes disposed on the reflective hating resistor film (2) for applying electricity to the reflective surface, wherein a maximum voltage drop in the electrodes (3a, 3b) with respect to a current feeding point on the electrodes is between 0.5% and 20% of a supply voltage. A mirror with a heater according to claim 1, characterised in that the opposing electrodes (3a,3b) are arranged on the reflective heating resistor film (2) so that the electrode interval at least near corners of the mirror base plate (1) is narrower than the electrode interval at a center portion thereof. A mirror with a heater according to claim 1 or 2, characterised in that the mirror base plate (1) has narrow angle portions and wide angle portions, and a temperature detection element (6) for temperature control is provided near an electrode end on the wide angle portion side of the opposing electrode, and the temperature control element (6) is disposed directly on the reflecting heating resistor (2). A mirror with a heater according to any preceding claim characterised in that the sheet resistivity distribution in the reflective heating resistor film (2) is uneven over the surface of the mirror base plate (1). 'A mirror with a heater according to any preceding claim characterised in that the mirror base plate has narrow angle portions and wide angle portions, and the voltage drop at the electrode end on the narrow angle portion side of each electrode (3a,3b) with respect to the current feeding point on the electode of the mirror base plate (1) is smaller than that at the electrode end on the wide angle portion side. A mirror with a heater according to any preceding claim characterised in that the mirror base plate (1) has narrow angle portions and wide angle portions, and the electrodes (3a,3b) have projections formed at least on the wide angle portion side of the opposing electrodes to suppress concentration of currents flowing into the wide angle portions.

Inventors: NAGAOKA MAKOTO, SUGIYAMA TETSUYA, TAZUNOKI HIROSHI, UEDA YOSHIYA, NAGAOKA, MAKOTO, SUGIYAMA, TETSUYA, TAZUNOKI, HIROSHI, UEDA, YOSHIYA
Applicants: PENTEL KK, PENTEL KABUSHIKI KAISHA
Application year: 1994
Main classification: B60S1
IPC codes: H05B3